Just wondering how to best integrate metta contemlation with sattipattana. Take the 3rd Foundation which is currently my "jam." When I "know" that my mind is not deluded, for example, would it be appropriate to contemplate "Metta?" That is how I'm approaching it, giving a bit more tenderness to a "wholesome" mind-state.

I don't know if there is a preferred way, but more typically I will move from metta to mindfulness than the other way around -- often metta settles/calms the mind, so when the metta is done, sometimes mindfulness is "just there" (w/o much hindrance). In going from mindfulness practice to metta, I suppose it would be useful to see as closely as possible what may actually be arising to cause that volition to shift to metta -- I know for me it could be a restlessness -- a desire to give my mind something more active to do. (YMMV of course!).

"...the practice is essentially a practice, and not a theory to be idly discussed...right view leaves unanswered many questions about the cosmos and the self, and directs your attention to what needs to be done to escape from the ravages of suffering." Thanissaro Bhikkhu, On The Path.

This is the Buddhas instruction for practicing metta with satipatthana:

"Monks, the establishing of mindfulness is to be practiced with the thought, 'I'll watch after myself.' The establishing of mindfulness is to be practiced with the thought, 'I'll watch after others.' When watching after yourself, you watch after others. When watching after others, you watch after yourself.

"And how do you watch after others when watching after yourself? Through cultivating [the practice], through developing it, through pursuing it. This is how you watch after others when watching after yourself.

"And how do you watch after yourself when watching after others? Through endurance, through harmlessness, through a mind of goodwill, & through sympathy. This is how you watch after yourself when watching after others.

"The establishing of mindfulness is to be practiced with the thought, 'I'll watch after myself.' The establishing of mindfulness is to be practiced with the thought, 'I'll watch after others.' When watching after yourself, you watch after others. When watching after others, you watch after yourself."

The Brahmaviharas or the "divine abidings," of loving kindness, compassion, joy and equanimity, should be the foundation of our awareness. - Ajahn Chah

There is no need to separate the two.

To study is to know the texts,
To practice is to know your defilements,
To attain the goal is to know and let go.

- Ajahn Lee Dhammadharo

With mindfulness immersed in the body
well established, restrained
with regard to the six media of contact,
always centered, the monk
can know Unbinding for himself.

- Ud 3.5

Ultimately, your meditation involves sustaining the knowing, followed by continuous letting go as you experience sense objects through the eyes, ears, nose, tongue, body and mind. It involves just this much and there is no need to make anything more out of it.